What was the first life form on earth?
It's surprisingly difficult to give you a short answer, because there is no fossil record of the first organism.
It is also quite difficult to state when a random strand of RNA or DNA could finally be considered alive. We think it was almost 4 billion years ago, or at least I think that is the first undisputed (relatively widely accepted) evidence for an organism, but clearly there must have been precursor(s) to this. (Organisms don't just appear as fully formed, replicating cells.)
This (http://www.physicsoftheuniverse.com/topics_life.html) may help give a better answer, or at least a fuller description of the difficulty.